An ion-sensitive field-effect transistor (ISFET) is a field-effect transistor used for measuring ion concentrations in solution. ISFET is a key device used in DNA sequencing. The ISFET device can detect pH changes in the solution through DNA polymerase synthesis. For example, the changes monitored by a sensing membrane (e.g., SiO2, Si3N4, Al2O3, Ta2O5, etc.) of the ISFET can be converted to electrical signals for measurements. More specifically, when the ion concentration (such as H+) changes, the current through the transistor will change accordingly. A voltage between substrate and oxide surfaces arises due to an ion sheath.